1. Field of the Invention
The present invention relates to lifting bodies and, more particularly, to both aerodynamic (e.g., wings, rotors, flaps, control surfaces) and hydrodynamic (e.g., submarine sails, bow-planes, stern appendages, propellers) lifting bodies with rapidly dissipated trailing longitudinal vortices.
2. Description of the Prior Art
Wing geometry plays an important role in the development of lift in both aerodynamic and hydrodynamic craft. When air or liquid flows over a wing surface, various turbulent flows occur. An increased understanding of the complex underlying structure in these various turbulent flows has led to some level of their control. The importance of controlling these turbulent flows can be appreciated by considering some examples.
In aircraft, the formation of strong longitudinal vortices, especially behind heavy aircraft developing large lift coefficients, causes large induced transverse wake velocities in the vortices. These vortices thus cause large disturbances after an aircraft has lifted off or landed, and the large airflow velocities in these disturbances can present extremely hazardous conditions for other aircraft subsequently landing or taking off. Accordingly, the Federal Aviation Administration has mandated minimum separation times between consecutive takeoffs and/or landings and minimum distances between aircraft in flight.
In ships, particularly those in which noise reduction is important, such as submarines, trailing vortices can be a significant noise source. For example, the ship's propeller blades can trail vortices, which is a source of noise. In addition, large vortices that can sometimes trail from a submarine sail in certain maneuvers not only create significant noise in themselves but also can affect the flow through the ship's propeller disc and create easily detected secondary noise.
FIGS. 1 to 4 illustrate the phenomenon of trailing vortices. When fluid flows around a lifting body such as a wing, a vortex field is created in the flow trailing the wing. With a rectangular wing 11, such as that shown in FIG. 1, travelling relative to a fluid at a velocity of U.sub.28, a single strong longitudinal vortex 12 trails the tip end 13 of the wing.
In a tapered wing 20, such as that shown in FIG. 2, the chord c varies along the length of the wing (along the y-axis), which can be appreciated from FIG. 3. This variation can be expressed as a function c(y). As is well known, the lift L generated by an airfoil depends on the length of the chord and the angle of attack .alpha. of the chord relative to the velocity vector U.sub.28, which will vary along the length of the wing in a twisted wing. Therefore, the lift L varies along the length of the wing, which can be expressed as a function L(y), and causes formation of a vortex field. This is essentially a "vortex sheet" S, consisting of a continuous series of vortices represented for illustrative purposes as individual vortices 21-28 extending along and from the trailing edge of the wing in the +x direction as shown in FIG. 4. The vorticity in this vortex sheet increases near the tip of the wing 20 (represented by larger circles in FIG. 4) and tends to combine downstream of the trailing edge of the wing, a process often visualized as "rolling up" of the vortex sheet. This is depicted schematically in FIG. 5.
The properties of the fluid flow within this trailing vortex field are generally known to those skilled in the art, and have been described extensively in many reference works, among them C. duP. Donaldson and A. J. Bilanin, "Vortex Wakes of Conventional Aircraft," AGARDograph No. 204, NATO Advisory Group for Aerospace Research and Development, Technical Editing and Reproduction Ltd., London, 1975, and Lee, H., "Computational and Experimental Study of Trailing Vortices," Ph.D. Thesis for Virginia Polytechnic Institute, 1983, available from University Microfilms International, Ann Arbor, Mich.
The fluid velocities induced by this vortex field S, and the resulting hazardous flight conditions or noise, depend on the strength and spatial distribution of the vortices that form as the vortex sheet rolls up. If the vortices can be properly modified, the vortex field will quickly dissipate.
Various approaches have been taken to modify the trailing vortices. One such approach, discussed in Donaldson and Bilanin, is shown in FIG. 6(a), in which a spline device depicted in FIG. 6(a') is located on the aircraft a certain distance behind the trailing edge of the wing for breaking up the vortex after it has formed. Another approach discussed in Donaldson and Bilanin is shown in FIG. 6(b), in which spoilers are positioned at specific locations on the wing.
However, the above-noted approaches add to the overall weight of the aircraft and negatively impact the aerodynamics of the wing by increasing drag. In addition, there is the possibility that even though the vortex sheet may be disturbed initially, it was not always certain whether the instability persisted further downstream of the trailing edge of the wing or if the flow reorganized into a tight vortex.
Donaldson & Bilanin also discuss redistribution of the lift along the wing so that more lift is generated at outboard sections of the wing (toward the wing tip) than toward the inboard sections. While this does not have the drag penalty associated with the other approaches, it does create substantial complexities in providing wing structure because generating more lift at the wing tip requires the wing to be made stronger and therefore heavier.
Other approaches have included modifying the wing planform providing notches in the trailing edge of the main part of the wing in an attempt to disrupt the formation of the vortex sheet.
None of these prior art approaches have resulted in significantly hampering the rolling-up process or significantly increasing the redistribution of the trailing wing-tip vortex sheet formed thereby.